Biogas: An Alternative Energy Source

03/01/2013 | Sarah K. Walls

Most professionals in the energy industry know about biomass; fewer of us are conversant with biogas. This commentary explains the basics of biogas, with a focus on its current use and future potential as a source of electrical power.

What Is Biogas?

Biogas is produced from anaerobic digestion of biodegradable organic matter, or “biomass.” Bacteria present in, or added to, the biomass ferments it anaerobically (without oxygen), through biochemical reactions. The constituents of biogas include methane (60% to 80%), carbon dioxide (20% to 40%), and trace amounts of hydrogen sulfide, nitrogen, and other impurities. Cleaning to remove impurities and moisture is necessary before biogas can be used as an energy source for certain alternatives, such as compressed natural gas. This purification is known as “upgrading” the biogas. Rendering biogas to be at least 98% methane produces a product known as “biomethane.” Biomethane performs identically to conventional fossil fuel natural gas, with all the same benefits and uses, and is one of the cleanest and most efficient alternative energy sources.

Major sources of biogas include municipal wastewater treatment plants, industrial waste treatment facilities, landfills, and agricultural sources such as manure and energy crops. In the past, such facilities used anaerobic digestion for stabilization, pathogen reduction, and volume reduction of wastes prior to disposal or land application. In many cases, biogas was simply flared.

Biogas now is developing into a significant alternative energy source. Using biogas to produce electricity satisfies several regulatory concerns at once. Greenhouse gas emissions are reduced, because the release of methane is prevented; green and renewable energy is produced; volumes of waste requiring storage and disposal are reduced.

Uses of Biomethane

Advantageous uses of biomethane include combined heat and power (CHP), boiler fuel, and injection into natural gas pipelines, along with use of compressed biomethane and liquefied biomethane for vehicle fuels. CHP systems, which produce both mechanical and thermal energy, can use biogas to produce electricity. The electricity produced can be used on-site, which reduces the facility’s electrical costs, or fed into mainstream power grids. CHP uses for biogas include internal combustion engines, combustion gas turbines, microturbines, fuel cells, and steam turbines.

In other countries, especially in Europe, where the cost of electricity is much higher than in the U.S., anaerobic digestion for biogas production is used widely. Anaerobic digestion facilities utilizing “energy crops,” such as corn, are already built or in progress in Germany, Sweden, Poland, Hungary, and Denmark. These facilities produce biogas, whereas in the U.S., energy crops are being used for production of fuel alcohol or ethanol. The production of biogas from energy crops is much more energy-efficient than the production of ethanol: For the same quantity of energy crop converted to alternative energy, the net energy value produced is greater with biogas.

The Kyoto Protocol has pushed other countries to establish renewable energy targets and promote development of renewable energy technologies. Germany was the first nation to enact feed-in-tariff (FIT) laws promoting biogas. The FIT legislation requires utilities to buy the electricity produced by biogas generators. Even German farmers can generate electricity from biogas and sell it to the grid.

Slow Start in the U.S.

Certain barriers have prevented broader use of biogas in the U.S. These barriers include less-than-favorable economics, lack of capital, technical complications, and air permitting delays. However, while the up-front capital investment necessary is high, benefits over the entire life cycle can exceed the initial costs.

The Village Creek Water Reclamation Facility in Fort Worth, Texas, is a prime example of using biogas to produce electricity. This cutting-edge municipal facility uses low-Btu methane biogas generated as a by-product of its anaerobic sludge digesters, six of which have been upgraded to take in high-strength liquid industrial wastes to supplement biogas production. The raw, untreated biogas, combined with biogas contributed by a nearby landfill, passes through a dehydrator prior to being burned as fuel in two 5.2-MW combustion turbines. The waste heat from the engines is combined with additional biogas in a duct burner to fire boilers, which produce steam. The steam is used to operate two steam turbines that operate two of the 1,000-horsepower blowers, which provide aeration for the activated biological treatment part of the facility. Once all units are operating, the facility will produce more biogas than is needed for its own operation and will have additional electricity to sell to the grid or trade for uses at other facilities.

Bright Future in the U.S.

The future of biogas in the U.S. will depend to a large extent upon the price of natural gas. Last year, President Obama’s Executive Order 13624 recognized the barriers that have led to under-investment in CHP and directed certain agencies and executive departments to convene stakeholders with the goal of accelerating investment in industrial energy efficiency, and in CHP in particular. A national goal was set of 40 GW of new, cost-effective CHP by the end of 2020—a 50% increase from today.

Biogas has much potential, and there has never been a better time for owners, state and federal government leaders, lenders, and utilities to work together to accomplish this challenging goal.

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